Altering properties of fuel compositions

ABSTRACT

A method for increasing the cetane number of a diesel fuel composition which contains a major proportion of a diesel base fuel is provided, in order to reach a target cetane number X. A fatty acid alkyl ester (FAAE) having a cetane number B greater than the cetane number A of the base fuel is added to the base fuel in an amount x, wherein x is less than the amount of the FAAE which would need to be added to the base fuel in order to achieve cetane number X if linear blending rules applied. The concentration of the FAAE in the overall fuel composition is preferably from 0.05 to 25% v/v.

FIELD OF THE INVENTION

The present invention relates to a method for increasing the cetanenumber of a diesel fuel composition.

BACKGROUND OF THE INVENTION

The cetane number of a fuel or fuel composition is a measure of its easeof ignition. With a lower cetane number fuel a compression ignition(diesel) engine tends to be more difficult to start and may run morenoisily when cold. There is a general preference, therefore, for adiesel fuel composition to have a high cetane number, and as suchautomotive diesel specifications generally stipulate a minimum cetanenumber. Many diesel fuel compositions contain cetane boost additives,also known as ignition improvers, to ensure compliance with suchspecifications.

SUMMARY OF THE INVENTION

Accordingly, a method of increasing the cetane number of a diesel fuelcomposition which contains a major proportion of a diesel base fuel isprovided, in order to reach a target cetane number X, said methodcomprising adding to the base fuel an amount x of a fatty acid alkylester (FAAE) having a cetane number B which is greater than the cetanenumber A of the base fuel, wherein x is less than the amount of the FAAEwhich would need to be added to the base fuel in order to achieve cetanenumber X if linear blending rules applied.

DETAILED DESCRIPTION OF THE INVENTION

Fatty acid alkyl esters (FAAEs), in particular fatty acid methyl esterscan be included, in diesel fuel compositions. An example of a FAAEincluded in diesel fuels is rapeseed methyl ester (RME). FAAEs aretypically derivable from biological sources and may be added for avariety of reasons, including to reduce the environmental impact of thefuel production and consumption process or to improve lubricity.

FAAEs often have higher cetane numbers than typical diesel base fuels.For example, the cetane number of soy methyl ester (SME) is generally˜55, whereas that for a typical European diesel base fuel is ˜51-55.

Following conventional fuel formulation principles, it would be expectedthat the cetane number of a base fuel/FAAE blend would vary linearlywith FAAE concentration. In other words, the addition of a FAAE to abase fuel having a lower cetane number would be expected to increase thecetane number of the fuel to an extent directly proportional to theamount of FAAE added.

It has now been discovered that FAAEs can produce a non-linear change incetane number when blended with diesel base fuels. The present inventionis able to provide a more optimised method for increasing the cetanenumber of a diesel fuel composition to reach a particular target value.

Accordingly in one embodiment of the present invention there is provideda method for increasing the cetane number of a diesel fuel compositionwhich contains a major proportion of a diesel base fuel, in order toreach a target cetane number X, which method comprises adding to thebase fuel an amount x of a fatty acid alkyl ester (FAAE) having a cetanenumber B greater than the cetane number A of the base fuel, wherein x isless than the amount of the FAAE which would need to be added to thebase fuel in order to achieve cetane number X if linear blending rulesapplied.

As described above, if linear blending rules applied then the cetanenumber of the base fuel/FAAE mixture would vary linearly with FAAEconcentration. If this were the case, it would then be straightforwardto calculate the amount of any given FAAE needed to increase the cetanenumber of the base fuel to reach the target X. However, it has now beenfound that, in particular at lower concentrations, a FAAE can actually“boost” the cetane number of a diesel base fuel above the level thatwould be expected if linear blending rules applied. This allows a loweramount of FAAE to be used to achieve any given target X, thus loweringany costs or other detrimental effects associated with inclusion of theFAAE.

Since it may be desirable to add a FAAE to a diesel fuel composition forother reasons, for example to reduce its environmental impact (includingto reduce emissions) and/or to improve its lubricity, the ability to usethe FAAE for the additional purpose of increasing the cetane number canprovide formulation advantages. Because the FAAE can cause anunexpectedly high cetane number, relatively small amounts may be used insome cases to replace, either partially or wholly, other cetaneimprovers which would otherwise be needed in the composition, thusreducing the overall additive levels in the composition and theirassociated costs.

In the present context, a “major proportion” of a base fuel meanstypically 80% v/v or greater, more suitably 90 or 95% v/v or greater,most preferably 98 or 99 or 99.5% v/v or greater. “Reaching” a targetcetane number can also embrace exceeding that number.

The FAAE may be added to the fuel composition as a blend (i.e. aphysical mixture), conveniently before the composition is introducedinto an internal combustion engine or other system which is to be run onthe composition. Other fuel components and/or fuel additives may also beincorporated into the composition, either before or after addition ofthe FAAE and either before or during use of the composition in acombustion system.

The amount of FAAE added will depend on the natures of the base fuel andFAAE in question and on the target cetane number. In general, the volumefraction v of FAAE in the resultant base fuel/FAAE mixture will be lessthan the volume fraction v′ which would be required if linear blendingrules applied, wherein v′ would be defined by the equationX=A+v′(B−A).The volume fractions v and v′ must each have a value between 0 and 1.When carrying out the method of the present invention the actual volumefraction of FAAE, v, is preferably at least 0.02 lower than the “linear”volume fraction v′, more preferably at least 0.05 or 0.08 or 0.1 lower,most preferably at least 0.2, 0.3 or 0.5 lower and in cases up to 0.6 or0.8 lower than v′. In absolute terms, the actual volume fraction v ispreferably 0.25 or less, more preferably 0.2 or less, yet morepreferably 0.15 or 0.1 or 0.07 or less. It may for example be from 0.01to 0.25, preferably from 0.05 to 0.25, more preferably from 0.05 or 0.1to 0.2.

Thus, as a result of carrying out the method of the present invention,the concentration of the FAAE in the overall fuel composition (or atleast in the base fuel/FAAE mixture) is preferably 25% v/v or less, morepreferably 20% v/v or less, yet more preferably 15 or 10 or 7% v/v orless. As a minimum it may be 0.05% v/v or greater, preferably 1% v/v orgreater, more preferably 2% or 5% v/v or greater, most preferably 7 or10% v/v or greater.

Fatty acid alkyl esters that may be used in the present context arepreferably the methyl esters, as renewable diesel fuels (so-called“biodiesel” fuels). They contain long chain carboxylic acid molecules(generally from 10 to 22 carbon atoms long), each having an alcoholmolecule attached to one end. Organically derived oils such as vegetableoils (including recycled vegetable oils) and animal fats can besubjected to a transesterification process with an alcohol (typically aC₁ to C₅ alcohol) to form the corresponding fatty esters, typicallymono-alkylated. This process, which is suitably either acid- orbase-catalysed, such as with the base KOH, converts the triglyceridescontained in the oils into fatty acid esters and free glycerol, byseparating the fatty acid components of the oils from their glycerolbackbone.

In the present invention, the FAAE may be any alkylated fatty acid ormixture of fatty acids. Its fatty acid component(s) are preferablyderived from a biological source, more preferably a vegetable source.They may be saturated or unsaturated; if the latter, they may have oneor more double bonds. They may be branched or un-branched. Suitably theywill have from 10 to 30, more suitably from 10 to 22 or from 12 to 22,carbon atoms in addition to the acid group(s)—CO₂H. A FAAE willtypically comprise a mixture of different fatty acid esters of differentchain lengths, depending on its source. For instance the commonlyavailable rapeseed oil contains mixtures of palmitic acid (C₁₆), stearicacid (C₁₈), oleic, linoleic and linolenic acids (C₁₈, with one, two andthree unsaturated carbon-carbon bonds respectively) and sometimes alsoerucic acid (C₂₂)—of these the oleic and linoleic acids form the majorproportion. Soybean oil contains a mixture of palmitic, stearic, oleic,linoleic and linolenic acids. Palm oil usually contains a mixture ofpalmitic, stearic and linoleic acid components.

The FAAE used in the present invention is preferably derived from anatural fatty oil, for instance a vegetable oil such as rapeseed oil,soybean oil, coconut oil, sunflower oil, palm oil, peanut oil, linseedoil, camelina oil, safflower oil, babassu oil, tallow oil or rice branoil. It may in particular be an alkyl ester (suitably the methyl ester)of rapeseed, soy, coconut or palm oil.

The FAAE is preferably a C₁ to C₅ alkyl ester, more preferably a methyl,ethyl or propyl (suitably iso-propyl) ester, yet more preferably amethyl or ethyl ester and in particular a methyl ester.

It may for example be selected from the group consisting of rapeseedmethyl ester (RME, also known as rape oil methyl ester or rape methylester), soy methyl ester (SME, also known as soybean methyl ester), palmoil methyl ester (POME), coconut methyl ester (CME) (in particularunrefined CME; the refined product is based on the crude but with someof the higher and lower alkyl chains (typically the C₆, C₈, C₁₀ C₁₆ andC₁₈) components removed) and mixtures thereof. In general it may beeither natural or synthetic, refined or unrefined (“crude”).

The FAAE suitably complies with specifications applying to the rest ofthe fuel composition, and/or to the base fuel to which it is added,bearing in mind the intended use to which the composition is to be put(for example, in which geographical area and at what time of year). Inparticular, the FAAE preferably has a flash point (IP 34) of greaterthan 101° C.; a kinematic viscosity at 40° C. (IP 71) of 1.9 to 6.0centistokes, preferably 3.5 to 5.0 centistokes; a density from 845 to910 kg/m³, preferably from 860 to 900 kg/m³, at 15° C. (IP 365, EN ISO12185 or EN ISO 3675); a water content (IP 386) of less than 500 ppm; aT95 (the temperature at which 95% of the fuel has evaporated, measuredaccording to IP 123) of less than 360° C.; an acid number (IP 139) ofless than 0.8 mgKOH/g, preferably less than 0.5 mgKOH/g; and an iodinenumber (IP 84) of less than 125, preferably less than 120 or less than115, grams of iodine (I₂) per 100 g of fuel. It also preferably contains(eg, by NMR) less than 0.2% w/w of free methanol, less than 0.02% w/w offree glycerol and greater than 96.5% w/w esters. In general it may bepreferred for the FAAE to conform to the European specification EN 14214for fatty acid methyl esters for use as diesel fuels.

The measured cetane number of the FAAE (ASTM D613) is suitably 55 orgreater, preferably 58 or 60 or 65 or even 70 or greater.

Two or more FAAEs may be added to the base fuel in accordance with thepresent invention, either separately or as a pre-prepared blend, so longas their combined effect is to increase the cetane number of theresultant composition to reach the target number X. In this case thetotal amount x′ of the two or more FAAEs must be less than the amount ofthat same combination of FAAEs which would need to be added to the basefuel in order to achieve the target cetane number X if linear blendingrules applied for both or all of the FAAEs.

The FAAE preferably comprises (i.e. either is or includes) RME or SME.

The FAAE may be added to the fuel composition for one or more otherpurposes in addition to the desire to increase cetane number, forinstance to reduce life cycle greenhouse gas emissions, to improvelubricity and/or to reduce costs.

The cetane number of a fuel composition may be determined in knownmanner, for instance using the standard test procedure ASTM D613 (ISO5165, IP 41) which provides a so-called “measured” cetane numberobtained under engine running conditions.

More preferably the cetane number may be determined using the morerecent and accurate “ignition quality test (IQT)” (ASTM D6890, IP498/03), which provides a “derived” cetane number based on the timedelay between injection and combustion of a fuel sample introduced intoa constant volume combustion chamber. This relatively rapid techniquecan be used on laboratory scale (ca. 100 ml) samples of a range ofdifferent diesel fuels.

Alternatively, cetane number may be measured by near infraredspectroscopy (NIR), as for example described in U.S. Pat. No. 5,349,188.This method may be preferred in a refinery environment as it can be lesscumbersome than for instance ASTM D613. NIR measurements make use of acorrelation between the measured spectrum and the actual cetane numberof the sample. An underlying model is prepared by correlating the knowncetane numbers of a variety of fuel samples (in this case, for example,diesel base fuels, FAAEs and/or blends thereof) with their near infraredspectral data.

The method of the present invention preferably results in a diesel fuelcomposition which has a derived cetane number (IP 498/03) of 50 orgreater, more preferably of 51 or 52 or 53 or greater.

The method of the present invention may additionally or alternatively beused to adjust any property of the diesel fuel composition which isequivalent to or directly associated with cetane number.

The diesel base fuel used in the composition may be any known dieselbase fuel, and it may itself comprise a mixture of diesel fuelcomponents. It will preferably have a sulfur content of at most 2000ppmw (parts per million by weight). More preferably it will have a lowor ultra low sulfur content, for instance at most 500 ppmw, preferablyno more than 350 ppmw, most preferably no more than 100 or 50 or even 10ppmw, of sulfur. The resultant mixture of the base fuel and the FAMEwill also preferably have a sulfur content within these ranges.

In some cases it may be preferred that the base fuel is not a sulfurfree (“zero sulfur”) fuel.

Typical diesel fuel components comprise liquid hydrocarbon middledistillate fuel oils, for instance petroleum derived gas oils. Such basefuel components may be organically or synthetically derived. They willtypically have boiling points within the usual diesel range of 150 to400° C., depending on grade and use. They will typically have densitiesfrom 0.75 to 0.9 g/cm³, preferably from 0.8 to 0.86 g/cm³, at 15° C. (IP365) and measured cetane numbers (ASTM D613) of from 35 to 80, morepreferably from 40 to 75 or 70. Their initial boiling points willsuitably be in the range 150 to 230° C. and their final boiling pointsin the range 290 to 400° C. Their kinematic viscosity at 40° C. (ASTMD445) might suitably be from 1.5 to 4.5 centistokes.

Such fuels are generally suitable for use in a compression ignition(diesel) internal combustion engine, of either the indirect or directinjection type.

Again, the fuel composition which results from carrying out the presentinvention will also preferably fall within these general specifications.In particular, its measured cetane number will preferably be from 45 to70 or 80, more preferably from 50 to 65 or at least greater than 50 oreven 53 or 55 or 57.

A petroleum derived gas oil may be obtained from refining and optionally(hydro)processing a crude petroleum source. It may be a single gas oilstream obtained from such a refinery process or a blend of several gasoil fractions obtained in the refinery process via different processingroutes. Examples of such gas oil fractions are straight run gas oil,vacuum gas oil, gas oil as obtained in a thermal cracking process, lightand heavy cycle oils as obtained in a fluid catalytic cracking unit andgas oil as obtained from a hydrocracker unit. Optionally a petroleumderived gas oil may comprise some petroleum derived kerosene fraction.

Such gas oils may be processed in a hydrodesulfurisation (HDS) unit soas to reduce their sulfur content to a level suitable for inclusion in adiesel fuel composition.

In one embodiment of the present invention, the base fuel may be orcontain another so-called “biodiesel” fuel component, such as an alcohol(in particular methanol or ethanol) or other oxygenate or a vegetableoil or vegetable oil derivative.

It may be or contain a Fischer-Tropsch derived fuel, in particular aFischer-Tropsch derived gas oil. Such fuels are known and in use indiesel fuel compositions. They are, or are prepared from, the synthesisproducts of a Fischer-Tropsch condensation reaction, as for example thecommercially used gas oil obtained from the Shell Middle DistillateSynthesis (Gas To Liquid) process operating in Bintulu (Malaysia).

The diesel fuel composition which results from the method of the presentinvention may if desired contain no, or only low levels of, other cetaneimproving (ignition improving) additives such as 2-ethylhexyl nitrate(EHN). In other words, the present invention embraces the use of a FAAEin a diesel fuel composition for the purpose of reducing the level ofcetane improving additives in the composition. As described above, theamount of FAAE used to achieve a given reduction in additive level willbe less than the amount that would be necessary if linear blending ofthe FAAE and base fuel applied.

In this context, “use” of a FAAE in a fuel composition meansincorporating the FAAE into the composition, typically as a blend (i.e.a physical mixture) and optionally with one or more other fuelcomponents (such as diesel base fuels) and optionally with one or morefuel additives. The FAAE is conveniently incorporated before thecomposition is introduced into an engine or other combustion systemwhich is to be run on the fuel composition. Instead or in addition theuse may involve running a diesel engine on the fuel compositioncontaining the FAAE, typically by introducing the composition into acombustion chamber of the engine.

The term “reducing” embraces reduction to zero; in other words, the FAAEmay be used to replace one or more cetane improving additives eitherpartially or completely. The reduction may be as compared to the levelof the relevant additive(s) which would otherwise have been incorporatedinto the fuel composition in order to achieve a desired target cetanenumber, for instance in order to meet government fuel specifications orconsumer expectations. Thus the FAAE can help in reducing the overalladditive levels in the composition and their associated costs.

Preferably the FAAE is used to reduce the w/w concentration of therelevant additive(s) in the fuel composition by at least 10%, morepreferably by at least 20 or 30%, yet more preferably by at least 50 or70 or 80 or even 90% or, as described above, by 100%.

It may for instance be used to replace cetane improving additive(s) toan extent that the concentration of cetane improving additives remainingin the fuel composition is 300 ppmw or less, preferably 200 ppmw orless, more preferably 100 or 50 ppmw or less. Most preferably it may beused to replace cetane improving additive(s) substantially entirely, thefuel composition being nearly or essentially free of such additives andcontaining for example 10 or 5 ppmw or less, preferably 1 ppmw or less,of cetane improving additives.

(All additive concentrations quoted in this specification refer, unlessotherwise stated, to active matter concentrations by mass. The term“cetane improving additive” refers to additives, other than the FAAE,which can increase the cetane number of a fuel or otherwise improve itsignition quality.)

Subject to the above, a diesel fuel composition prepared according tothe present invention may contain other components in addition to thediesel base fuel and the FAAE. Typically such components will beincorporated in fuel additives. Examples include detergents such aspolyolefin substituted succinimides or succinamides of polyamines, forinstance polyisobutylene succinimides or polyisobutylene aminesuccinamides, aliphatic amines, Mannich bases or amines and polyolefin(e.g. polyisobutylene) maleic anhydrides. Succinimide dispersantadditives are described for example in GB-A-960493, EP-A-0147240,EP-A-0482253, EP-A-0613938, EP-A-0557516 and WO-A-98/42808. Particularlypreferred are polyolefin substituted succinimides such aspolyisobutylene succinimides.

The additives may contain other components in addition to the detergent.Examples are lubricity enhancers such as amide-based additives;dehazers, e.g. alkoxylated phenol formaldehyde polymers; anti-foamingagents (e.g. polyether-modified polysiloxanes); anti-rust agents (e.g. apropane-1,2-diol semi-ester of tetrapropenyl succinic acid, orpolyhydric alcohol esters of a succinic acid derivative, the succinicacid derivative having on at least one of its alpha-carbon atoms anunsubstituted or substituted aliphatic hydrocarbon group containing from20 to 500 carbon atoms, e.g. the pentaerythritol diester ofpolyisobutylene-substituted succinic acid); corrosion inhibitors;reodorants; anti-wear additives; anti-oxidants (e.g. phenolics such as2,6-di-tert-butylphenol, or phenylenediamines such asN,N′-di-sec-butyl-p-phenylenediamine); metal deactivators; andcombustion improvers.

Where the fuel composition contains one or more ignition improvers(cetane improvers), these may be selected from for example 2-ethylhexylnitrate (EHN), cyclohexyl nitrate, di-tert-butyl peroxide and thosedisclosed in U.S. Pat. No. 4,208,190 at column 2, line 27 to column 3,line 21.

The fuel composition will suitably contain only a minor amount of suchadditives. Thus unless otherwise stated, the (active matter)concentration of each such additional component in the overall fuelcomposition is preferably up to 1% w/w, more preferably in the rangefrom 5 to 1000 ppmw, advantageously from 75 to 300 ppmw, such as from 95to 150 ppmw.

It is particularly preferred that a lubricity enhancer be included inthe fuel composition, especially when it has a low (e.g. 500 ppmw orless) sulfur content. The lubricity enhancer is conveniently present ata concentration from 50 to 1000 ppmw, preferably from 100 to 1000 ppmw,based on the overall fuel composition.

Additives may be added at various stages during the production of a fuelcomposition; those added at the refinery for example might be selectedfrom anti-static agents, pipeline drag reducers, flow improvers (e.g.ethylene/vinyl acetate copolymers or acrylate/maleic anhydridecopolymers) and wax anti-settling agents. When carrying out the methodof the present invention, the diesel base fuel may already contain suchrefinery additives. Other additives may be added downstream of therefinery.

The method of the present invention may form part of a process for, orbe implemented using a system for, controlling the blending of a fuelcomposition, for example in a refinery. Such a system will typicallyinclude means for introducing a FAAE and a diesel base fuel into ablending chamber, flow control means for independently controlling thevolumetric flow rates of the FAAE and the base fuel into the chamber,means for calculating the volume fraction of the FAAE needed to achievea desired target cetane number input by a user into the system and meansfor directing the result of that calculation to the flow control meanswhich is then operable to achieve the desired volume fraction in theproduct composition by altering the flow rates of its constituents intothe blending chamber.

In order to calculate the required volume fraction, a process or systemof this type will suitably make use of known cetane numbers for the basefuel and FAAE concerned, and conveniently also a model predicting thecetane number of varying concentration blends of the two according tolinear blending rules. The process or system may then, according to thepresent invention, select and produce a FAAE volume fraction lower thanthat predicted by the linear blending model to be necessary. It may usea so-called quality estimator which will provide, using a model, areal-time prediction of the cetane number of each resulting blend fromavailable raw process measurements, such as for example the NIR measuredcetane numbers and the volumetric flow rates of the constituents. Morepreferably such a quality estimator is calibrated on-line by making useof, for example, the method described in WO-A-02/06905.

The method of the present invention may thus conveniently be used toautomate, at least partially, the formulation of a diesel fuelcomposition, preferably providing real-time control over the relativeproportions of the FAAE and base fuel incorporated into the composition,for instance by controlling the relative flow rates of the constituents.

In accordance with the present invention, “use” of a FAAE in the waysdescribed above may also embrace supplying a FAAE together withinstructions for its use in a diesel fuel composition to increase thecetane number to a particular target and/or to improve the ignitionquality of the composition and/or to reduce the level of cetaneimproving additives in the composition. The FAAE may be supplied as acomponent of a formulation suitable and/or intended for use as a dieselfuel additive, in which case the FAAE may be included in the formulationfor the purpose of influencing its effects on the ignition quality of adiesel fuel composition.

Another embodiment of the present invention provides a diesel fuelcomposition prepared, or preparable, using a method according to thefirst aspect. This composition contains a major proportion of a dieselbase fuel which preferably has a low sulfur content (for example, lessthan 400 or preferably 300 ppmw) and/or a measured cetane number (ASTMD613) of from 48 to 52.

The present invention also provides a method of operating a dieselengine, and/or a vehicle which is driven by a diesel engine, whichmethod involves introducing into a combustion chamber of the engine adiesel fuel composition according to the second aspect. The fuelcomposition may be used in this way for the purpose of improving ease offuel ignition during use of the engine.

Preferred features of other embodiments of the present invention may beas described above in connection with any one of the embodiments of theinvention.

Other features of the present invention will become apparent from thefollowing examples. Generally speaking the present invention extends toany novel one, or any novel combination, of the features disclosed inthis specification (including the accompanying claims). Moreover, unlessstated otherwise, any feature disclosed herein may be replaced by analternative feature serving the same or a similar purpose.

EXAMPLES

These Examples demonstrate the effects of fatty acid alkyl esters, inparticular fatty acid methyl esters, on the cetane numbers of varioustypical diesel base fuels.

The fatty acid methyl esters (FAMEs) tested were rapeseed methyl ester(RME) and soy methyl ester (SME).

The base fuels tested were a typical German specification sulfur-free(“zero sulfur”) diesel fuel F1, a US specification diesel fuel F2, asummer grade European specification ultra low sulfur diesel fuel F3(without additives) and a diesel fuel F4 which was the same as F3 butcontained a standard refinery treatment additive (single dose treatrate; contained no cetane improvers).

The specifications for the base fuels F1 to F3 are shown in Table A.TABLE A Base Base Base fuel fuel fuel Fuel property Test method F1 F2 F3Density @ 15° C. (kg/m³) IP 365 837.1 843.7 830.4 Measured cetane numberASTM D613 50.4 50.3 53.9 Kinematic viscosity @ IP 71 2.851 3.058 2.50640° C. (centistokes) Distillation (° C.) IP 123 IBP 175.2 211.5 168 10%recovery 211.1 233.2 201 20% 227.5 245.1 220 30% 242.6 256 240 40% 257.3266.3 256 50% 271.1 275.1 269 60% 284.8 284.1 280.5 70% 299.3 294.2291.5 80% 316.1 305.8 303.5 90% 337.4 322.2 319.5 95% 355.2 337.5 335.5FBP 365.9 349.3 349 Cold filter plugging IP 309 −29 −12 −18 point (° C.)Cloud point (° C.) IP 219 −9 −11.8 −11 Flash point (° C.) IP 34 63.592.5 63 Sulfur (ppmw) ASTM D2622 9 290 27 Iodine number IP 84 8 9 18.12Acid number (total) IP 139 0.02 0.04 0.05 (mgKOH/g)

Various blends of the fatty acid methyl esters (FAMEs) and the basefuels were prepared, to assess the effect of FAME concentration on theignition quality of the resultant fuels.

Derived cetane numbers were determined for most samples, using theignition quality test (IQT) method IP 498/03. For some samples, measured(engine) cetane numbers were also obtained according to the CFR CetaneEngine method, ASTM D613.

Example 1 Effect of RME on Cetane Number

The effect of RME on both the measured and the derived cetane numbers ofvarious diesel base fuels was assessed as described above. The resultsare shown in Tables 1 to 4 for the base fuels F1 to F4 respectively.

The derived cetane number for the neat RME was 58.1. TABLE 1 RME in basefuel F1 RME concentration 0 (base (% v/v) fuel alone) 2 5 7 10 15 20 2530 Measured 50.4 X 52.8 X 50.4 X 52.4 X X cetane number Derived 51.450.9 51.1 51.5 52.4 52.5 53.2 54.1 54.9 cetane number(X = not measured)

TABLE 2 RME in base fuel F2 RME concentration 0 (base (% v/v) fuelalone) 2 5 7 10 15 20 25 30 Measured 50.3 X 51.9 X 52.8 X 54.3 X Xcetane number Derived 48 49 50.2 50.3 51 52.7 54 55.1 55.1 cetane number(X = not measured)

TABLE 3 RME in base fuel F3 RME concentration 0 (base (% v/v) fuelalone) 2 5 7 10 15 20 25 30 Measured 53.9 X 55.1 X 55.4 X 56.1 X Xcetane number Derived 52.5 53.3 54.4 54.9 54.8 55 55.7 54.8 55.7 cetanenumber(X = not measured)

TABLE 4 RME in base fuel F4 RME concentration 0 (base (% v/v) fuelalone) 2 5 7 10 15 20 25 30 Measured 54.2 X 54.3 X 55.4 X 55.3 X Xcetane number Derived 52.5 53.3 53.9 53.5 54.7 54.5 55.7 54.8 54.8cetane number(X = not measured)

These data show a non-linear change in cetane number with RMEconcentration, for all the base fuels tested. In particular, they show amarked “boost” in cetane number at lower RME concentrations, such as 20%v/v or below. Thus in this regime, for any given RME concentration thecetane number of the base fuel/RME blend is higher than linear blendingrules would have predicted. Correspondingly, in order to achieve anygiven target cetane number, a lower amount of the RME is needed than iflinear blending rules applied.

The trend is highlighted by the higher accuracy IQT data.

The presence of the refinery additive in base fuel F4 appears to have nosignificant impact on the ability of the RME to enhance cetane number.

For the zero sulfur fuel F1, it appears that slightly higher FAMEconcentrations (e.g. 10% v/v or greater) are needed to achieve such asignificant cetane number boost.

Example 2 Effect of SME on Cetane Number

The effect of SME (soy methyl ester) on both the measured and thederived cetane numbers of the four base fuels was assessed as describedabove. The results are shown in Tables 5 to 8.

The derived cetane number for the neat SME was 71.4. TABLE 5 SME in basefuel F1 SME concentration 0 (base (% v/v) fuel alone) 2 5 7 10 15 20 2530 Measured 50.4 X 55.5 X 57.4 X 58.7 X X cetane number Derived 51.454.5 56.8 57 57.9 59 59.9 62.2 60 cetane number(X = not measured)

TABLE 6 SME in base fuel F2 SME concentration 0 (base (% v/v) fuelalone) 2 5 7 10 15 20 25 30 Measured 50.3 X 55.5 X 56 X 56.7 X X cetanenumber Derived 48 51.7 54.6 56.5 55.5 57.5 57.6 59.3 59.2 cetane number(X = not measured)

TABLE 7 SME in base fuel F3 SME concentration 0 (base (% v/v) fuelalone) 2 5 7 10 15 20 25 30 Measured 53.9 X 57.2 X 57.8 X 58.9 X Xcetane number Derived 52.5 55.7 57.8 57.2 57.9 57.5 60.2 57.9 58.5cetane number(X = not measured)

TABLE 8 SME in base fuel F4 SME concentration 0 (base (% v/v) fuelalone) 2 5 7 10 15 20 25 30 Measured 54.2 X 57.6 X 58.6 X 60 X X cetanenumber Derived 52.5 54.7 58.3 57.1 59 59.1 61 59.7 59.4 cetane number(X = not measured)

Again these data show a non-linear boost in cetane number at lower SMEconcentrations, for all base fuels. As for RME, there is nostatistically significant difference in this effect between theadditivated (F4) and unadditivated (F3) fuels.

It can therefore be seen that a target increase in cetane number may beachieved, for the base fuels, by incorporating a FAAE in an amountsmaller than the amount that would be needed if linear blending applied.For example in the case of the SME/F2 blends (see Table 6), a targetcetane number of 51.7 can be achieved using only 2% v/v SME, whereas iflinear blending applied, one would expect 15.8% v/v of SME to be neededto achieve the same cetane number. Similarly a target cetane number of56.5 can be achieved using only 7% v/v SME, whereas linear blendingrules would predict that 36.3% v/v of SME would be needed to achievethis effect. (These figures are derived cetane numbers, measured by theIP 498/03 method.)

1. A method for increasing the cetane number of a diesel fuelcomposition which contains a major proportion of a diesel base fuel, inorder to reach a target cetane number X, said method comprising addingto the base fuel an amount x of a fatty acid alkyl ester (FAAE) having acetane number B which is greater than the cetane number A of the basefuel, wherein x is less than the amount of the FAAE which would need tobe added to the base fuel in order to achieve cetane number X if linearblending rules applied.
 2. The method of claim 1 wherein the volumefraction of FAAE added to the base fuel, v, is at least 0.05 lower thanthe volume fraction v′ which would be needed if linear blending rulesapplied.
 3. The method of claim 1 wherein the concentration of the FAAEin the overall fuel composition is from 0.05 to 25% v/v.
 4. The methodof claim 2 wherein the concentration of the FAAE in the overall fuelcomposition is from 0.05 to 25% v/v.
 5. The method of claim 3 whereinthe concentration of the FAAE in the overall fuel composition is from 1to 15% v/v.
 6. The method of claim 4 wherein the concentration of theFAAE in the overall fuel composition is from 1 to 15% v/v.
 7. The methodof claim 1 wherein the FAAE is a fatty acid methyl, ethyl or iso-propylester.
 8. The method of claim 1 wherein the FAAE is selected from thegroup consisting of rapeseed methyl ester, soy methyl ester, palm oilmethyl ester, coconut methyl ester and mixtures thereof.
 9. The methodof claim 1 wherein the diesel fuel composition contains less than 50ppmw of other cetane improving additives.
 10. The method of claim 1wherein the derived cetane number (IP 498/03) of the diesel fuelcomposition, as a result of use of the FAAE in the composition, is 50 orgreater.
 11. The method of claim 1 wherein the FAAE is derived from anatural fatty oil.
 12. The method of claim 8 wherein the concentrationof the FAAE in the overall fuel composition is from 0.05 to 25% v/v. 13.The method of clam 12 wherein the concentration of the FAAE in theoverall fuel composition is from 1 to 15% v/v.
 14. A method forincreasing the cetane number of diesel fuel composition which contains amajor proportion of a diesel base fuel, in order to reach a targetcetane number X, said method comprising adding to the base fuel anamount x of a fatty acid alkyl ester (FAAE) having a cetane number whichis greater than the cetane number A of the base fuel, wherein x is lessthan the amount of the FAAE which would need to be added to the basefuel in order to achieve octane number x if linear blending rulesapplied, said FAAE is selected from the group consisting of rapeseedmethyl ester, soy methyl ester, palm oil methyl ester, coconut methylester and mixtures thereof, wherein the concentration of the FAAE in theoverall fuel composition is from 1 to 15% v/v.